Efficiency optimization control method for double-direct-current bus partial power conversion parallel battery energy storage system

Abstract

The invention provides a partial power conversion parallel battery pack energy storage system efficiency optimization control method based on bus voltage regulation. The power conversion system of the partial power conversion parallel battery pack energy storage system comprises an equalization unit for controlling charging and discharging power and equalizing the state of charge (SOC) of a battery pack, and a bus voltage control unit for adjusting the voltage of an equalization bus. According to the control method, the characteristic of slow SOC change of the battery module of the energy storage system is considered, the operation efficiency characteristic of the equalization unit is combined, the voltage value of the equalization bus is adjusted through an optimization algorithm, the voltage of the input port and the voltage of the output port of the equalization unit are matched, the equalization unit is made to be in a low-loss operation interval, and therefore the efficiency value of the whole system is optimized. According to the method, by comparing the change of the total efficiency of the system before and after the voltage value of the equalization bus changes, the voltage value of the equalization bus is automatically adjusted to automatically track the optimal efficiency value of the system in real time, and the energy storage system is ensured to always work in a high-efficiency state. The method improves the operation efficiency of the partial power conversion parallel battery pack energy storage system.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronic direct current (DC-DC) converters, and in particular relates to an efficiency optimization control method for a dual DC bus partial power conversion parallel battery energy storage system based on bus voltage regulation. Background Art

[0002] In recent years, battery energy storage systems have been widely researched due to their high energy efficiency, rapid response, and ability to effectively mitigate the impact of renewable energy on the power grid. Due to the limited capacity and low voltage of individual cells, battery energy storage systems typically consist of multiple cells connected in series and parallel to meet application requirements. Parallel expansion offers significant flexibility in battery capacity expansion.

[0003] However, in a centralized energy storage architecture, direct parallel expansion solutions will bring about problems such as parallel balancing of battery modules and circulating current suppression. Introducing DC / DC conversion in the parallel branches of the battery pack to form a common DC bus string energy storage system can achieve parallel balancing of battery clusters and suppress circulating current, but it increases full-power conversion losses and is not conducive to improving the overall efficiency of the system. Although converter efficiency can be improved through converter topology and control optimization, the efficiency improvement space is limited, and the converter voltage and current stress are high, increasing system cost.

[0004] Partial power processing technology can effectively address the issues presented by full-power technology. By creating an electrical connection between the source and the load through a partial power structure, the majority of the power can be transferred directly between them, with the converter processing only a small portion of the power. This technology also allows for energy storage charging and discharging power control. However, the converter components experience high voltage stress, increasing costs, and improvements are still needed.

[0005] The dual DC bus architecture introduces a converter to provide a balanced DC bus for the partially converted parallel battery energy storage system, thereby reducing converter voltage stress and lowering costs. The added balanced bus voltage provides a new degree of control freedom for system efficiency optimization.

[0006] The present invention provides an efficiency optimization control method for a partial power parallel energy storage system based on bus voltage regulation, which reduces converter operation losses and improves the charging and discharging efficiency of the energy storage system without changing the topology structure and converter control method. Summary of the Invention

[0007] The present invention provides a partial power parallel battery group energy storage system efficiency optimization control algorithm for a dual-bus partial power conversion parallel battery group energy storage system, thereby realizing parallel battery group balancing and efficiency optimization control.

[0008] The topology diagram of the partial power conversion parallel battery storage system involved in the invention is as follows: Figure 1As shown in the figure, it mainly consists of five parts: a DC bus, a balancing bus, n parallel battery packs, n balancing units and a bus voltage control unit.

[0009] Each parallel branch battery pack is connected in series with the corresponding balancing unit and then connected in parallel to the DC bus. The output terminals of the balancing units are connected in parallel to the balancing bus. The input terminal of the bus voltage control unit is directly connected to the DC bus, and the output terminal is connected to the balancing bus.

[0010] Each parallel branch of the energy storage system uses a partial power conversion structure, in which the majority of the power is transferred directly between the DC bus and the battery pack. Only the power generated by the difference between the DC bus voltage and the battery pack terminal voltage is processed by the balancing unit. By controlling the current of the balancing unit, the battery pack's charge and discharge management and SOC balancing can be achieved. At the same time, the bus voltage control unit is responsible for regulating the output voltage of each balancing unit (balancing bus voltage). This not only reduces the voltage at the output of the balancing unit but also provides a path for energy transmission during the balancing process. At the same time, the efficiency of the entire system can be changed by adjusting the voltage of the balancing bus.

[0011] In order to optimize the efficiency of the entire parallel battery energy storage system, the present invention provides a partial power energy storage system efficiency optimization control method based on bus voltage regulation, the specific steps are as follows:

[0012] Step 1: Initialize system settings, set the balanced bus initial voltage, voltage adjustment step size ΔU, voltage adjustment time interval Δt, counter upper limit s, and voltage adjustment direction Mode.

[0013] Step 2: Select the voltage regulation direction according to the voltage regulation direction Mode. If Mode = 1, the balanced bus voltage increases by a step size ΔU and waits for the voltage regulation time interval Δt; if Mode = 2, the balanced bus voltage decreases by a step size ΔU and waits for the voltage regulation time interval Δt.

[0014] Step 3: Evaluate system efficiency. If the energy storage system operates in charging mode, the system efficiency is calculated using formula (1); if the energy storage system operates in discharging mode, the system efficiency is calculated using formula (2); if the system efficiency is evaluated indirectly, the system efficiency can be calculated using formula (4): Bus voltage control unit input port power P d Indirect assessment.

[0015] Step 4: Determine the change in system efficiency. If the system efficiency increases, the counter is cleared and the process goes to step 2. If the system efficiency decreases, the counter value is increased by 1 and the process goes to step 5.

[0016] Step 5: Determine whether the counter is equal to the upper limit s. If so, switch the voltage regulation direction, reset the counter, and go to step 2.

[0017] Furthermore, a method for optimizing the efficiency of a dual DC bus partial power conversion parallel battery energy storage system is provided, wherein step 2 is specifically as follows:

[0018] Mode is the direction of bus voltage regulation. Mode = 1 indicates an upward adjustment for the bus voltage; Mode = 2 indicates a downward adjustment for the bus voltage. Given the slow changes in battery module SOC, a longer voltage regulation interval Δt can be set to avoid frequent bus voltage adjustments and facilitate accurate evaluation of system efficiency.

[0019] Furthermore, a method for optimizing the efficiency of a dual DC bus partial power conversion parallel battery energy storage system is provided, wherein step 3 is specifically as follows:

[0020] (1) Directly evaluate system efficiency.

[0021] The efficiency of the energy storage system in discharge mode is calculated as follows:

[0022]

[0023] The efficiency of the energy storage system in charging mode is calculated as follows:

[0024]

[0025] Among them, η j is the efficiency of the jth balancing unit, η o is the efficiency of the bus voltage control unit, η all is the system efficiency, P s and P isj are the total charge and discharge power of the battery and the input port power of each balancing unit. In formula (1), K j is the power of the jth battery module discharge power input to the DC bus through the balancing unit and the bus voltage control unit; in formula (2), K j It is the charging power of the DC bus output power flowing into the j-th battery pack through the balancing unit and the bus voltage control unit.

[0026] G j is the transformation ratio of the jth subunit, calculated by formula (3):

[0027]

[0028] Among them, U bj is the battery module port voltage of the jth parallel branch, U isj is the voltage at the input port of the j-th parallel branch balancing unit.

[0029] (2) Indirect evaluation of system efficiency can be determined by the bus voltage control unit input port power.

[0030] The system efficiency of the energy storage system in charging or discharging mode is indirectly evaluated by the following formula:

[0031] P d =U id *I id (4)

[0032] Among them, P d is the power of the bus voltage control unit input port, I id and U id are the voltage and current of the bus voltage control unit input port respectively. d <0, the system works in charging mode. If P d >0, the system works in discharge mode. When the charging and discharging power of the energy storage system remains unchanged, P d Can reflect changes in system efficiency.

[0033] Furthermore, a method for optimizing the efficiency of a dual DC bus partial power conversion parallel battery energy storage system is provided, wherein step 4 is specifically as follows:

[0034] If the system efficiency is directly calculated and evaluated using formula (1) and formula (2), set η all_last is the result of the last system efficiency calculation. If η all >η all_last , system efficiency increases; η all <η all_last , system efficiency is reduced.

[0035] If the bus voltage control unit input port power is calculated using formula (4) to indirectly determine the system efficiency, set P d_last is the power value calculated last time. d <P d_last , system efficiency increases; P d >P d_last , system efficiency is reduced.

[0036] Compared with the prior art, the present invention has at least the following beneficial effects:

[0037] A method for optimizing the efficiency of a dual-DC bus partial power conversion parallel battery energy storage system is proposed. By changing the balanced bus voltage value, the efficiency of each converter is changed, so that the system operates at optimal efficiency.

[0038] An efficiency optimization control method for a dual DC bus partial power conversion parallel battery energy storage system can be used to directly and indirectly evaluate system efficiency and is applicable to systems with different control configurations.

[0039] An efficiency optimization control method for a partial power energy storage system based on bus voltage regulation fully utilizes the characteristic of slow state-of-charge (SOC) change of battery modules and sets a long waiting time Δt and a counter upper limit to ensure stable system control.

[0040] In summary, the method of the present invention changes the balanced bus voltage value, determines the system efficiency change, and automatically tracks the system's optimal efficiency. This method improves the efficiency of the partial power conversion parallel battery storage system without changing the original internal DC-DC converter topology. It is applicable to multi-battery parallel energy storage systems and has universal applicability.

[0041] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 A topological diagram applicable to this patent;

[0043] Figure 2 The topological equivalent circuit diagram applicable to this patent;

[0044] Figure 3 This is the control block diagram of the DAB converter of the bus voltage control unit;

[0045] Figure 4 This is the control block diagram of the SR-DAB converter group of the equalization unit;

[0046] Figure 5 This is a control flow chart of the automatic optimization algorithm of the present invention;

[0047] Figure 6 The following is a graph showing the operation of each converter and the change trend of the balanced bus voltage when the system is in steady-state operation under the efficiency optimization algorithm when the charging current is 4A;

[0048] Figure 7 This is the automatic optimization graph of the efficiency optimization algorithm after the charging power changes under charging conditions;

[0049] Figure 8 The following is a graph showing the operation and balanced bus voltage change trends of each converter when the system is in steady-state operation under the efficiency optimization algorithm when the discharge current is -4A;

[0050] Figure 9 This is the automatic optimization diagram of the efficiency optimization algorithm after the discharge power changes under discharge conditions;

[0051] Figure 10 The comparison of system efficiency with and without the optimization algorithm when the current is 4A;

[0052] Figure 11 The comparison of system efficiency with and without the optimization algorithm when the current is -4A; Specific implementation methods

[0053] The partial power topology structure adopted by the present invention is a series partial power conversion topology structure, and the applicable partial power conversion battery energy storage system topology structure is as follows: Figure 1 The system mainly consists of five parts: a DC bus, a balancing bus, n parallel battery packs, n balancing units and a bus voltage control unit.

[0054] The balancing unit and bus voltage control unit in the system topology are required to achieve bidirectional power flow to meet the system's operation requirements under different battery pack charge and discharge states. The balancing unit can use a series resonant dual active bridge converter (SR-DAB), while the bus voltage control unit can use a traditional dual active bridge converter (DAB). Each battery pack is connected in series with the corresponding balancing unit and then in parallel to the DC bus, while the outputs of the balancing units are connected in parallel. The input of the bus voltage control unit is directly connected to the DC bus, and the output is connected to the balancing unit.

[0055] The control block diagram of the bus voltage control unit is as follows: Figure 3 The automatic optimization algorithm of the present invention calculates the efficiency change and gives the balanced bus voltage reference value U od_ref , the given reference voltage U od_ref and the actual balanced bus voltage U od After the difference is made, the PI regulator generates the power reference value P of the optimization algorithm. o , and then the per-unit power value P is obtained by standardization. The power per-unit value P and the voltage gain ratio K of the converter are _DAB The extended phase-shift control algorithm is input together to calculate the inner phase-shift ratio D1 and the outer phase-shift ratio D2. The automatic optimization algorithm and the extended phase-shift control algorithm of the present invention constitute a dual-loop control structure of the bus voltage control unit.

[0056] The SOC balancing strategy of the present invention is: real-time estimation of the SOC of each parallel battery module, and calculation of the average value based on the SOC of each parallel battery module, setting the droop coefficient M, and then compensating the average charge and discharge current command of the balancing voltage according to the difference between the SOC of each battery module and the average SOC of the battery module, thereby achieving differentiated charging and discharging of different battery packs. Based on the single phase shift control of the SR-DAB converter, the SOC balancing control of the battery pack is added, such as Figure 4 shown.

[0057] Figure 5 This is the flow chart of the control part of the efficiency optimization algorithm. The specific control steps are as follows:

[0058] Step 1: Initialize system settings, set the balanced bus initial voltage, voltage adjustment step size ΔU, voltage adjustment time interval Δt, counter upper limit s, and voltage adjustment direction Mode.

[0059] Step 2: Select the voltage regulation direction according to the voltage regulation direction Mode. If Mode = 1, the balanced bus voltage increases by a step size ΔU and waits for the voltage regulation time interval Δt; if Mode = 2, the balanced bus voltage decreases by a step size ΔU and waits for the voltage regulation time interval Δt.

[0060] Step 3: Evaluate system efficiency. If the energy storage system operates in charging mode, the system efficiency is calculated using formula (1); if the energy storage system operates in discharging mode, the system efficiency is calculated using formula (2); if the system efficiency is evaluated indirectly, the system efficiency can be calculated using formula (4) Bus voltage control unit input port 0 power P d Indirect assessment.

[0061] Step 4: Determine the change in system efficiency. If the system efficiency increases, the counter is cleared and the process goes to step 2. If the system efficiency decreases, the counter value is increased by 1 and the process goes to step 5.

[0062] Step 5: Determine whether the counter is equal to the upper limit s. If so, switch the voltage regulation direction, reset the counter, and go to step 2.

[0063] Furthermore, a method for optimizing the efficiency of a dual DC bus partial power conversion parallel battery energy storage system is provided, wherein step 2 is specifically as follows:

[0064] Mode is the direction of bus voltage regulation. Mode = 1 indicates an upward adjustment for the bus voltage; Mode = 2 indicates a downward adjustment for the bus voltage. Given the slow changes in battery module SOC, a longer voltage regulation interval Δt can be set to avoid frequent bus voltage adjustments and facilitate accurate evaluation of system efficiency.

[0065] Furthermore, a method for optimizing the efficiency of a dual DC bus partial power conversion parallel battery energy storage system is provided, wherein step 3 is specifically as follows:

[0066] (1) Directly evaluate system efficiency.

[0067] The efficiency of the energy storage system in discharge mode is calculated as follows:

[0068]

[0069] The efficiency of the energy storage system in charging mode is calculated as follows:

[0070]

[0071] Among them, ηj is the efficiency of the jth balancing unit, η o is the efficiency of the bus voltage control unit, η all is the system efficiency, P s and P isj are the total charge and discharge power of the battery and the power of the input port j of the balancing unit. In formula (1), K j is the power of the jth battery module discharge power input to the DC bus through the balancing unit and the bus voltage control unit; in formula (2), K j It is the charging power of the DC bus output power flowing into the j-th battery pack through the balancing unit and the bus voltage control unit.

[0072] G j is the transformation ratio of the jth subunit, calculated by formula (3):

[0073]

[0074] Among them, U bj is the battery module port voltage of the jth parallel branch, U isj is the port voltage at the input port j of the j-th parallel branch balancing unit.

[0075] (2) Indirect evaluation of system efficiency can be determined by the bus voltage control unit input port 0 power.

[0076] The system efficiency of the energy storage system in charging or discharging mode is indirectly evaluated by the following formula:

[0077] P d =U id *I id (4)

[0078] Among them, P d is the power of bus voltage control unit input port 0, I id and U id are the voltage and current of bus voltage control unit input port 0 respectively. d <0, the system works in charging mode. If P d >0, the system works in discharge mode. When the charging and discharging power of the energy storage system remains unchanged, P d Can reflect changes in system efficiency.

[0079] Furthermore, a method for optimizing the efficiency of a dual DC bus partial power conversion parallel battery energy storage system is provided, wherein step 4 is specifically as follows:

[0080] If the system efficiency is directly calculated and evaluated using formula (1) and formula (2), set η all_last is the result of the last system efficiency calculation. If η all >ηall_last , system efficiency increases; η all <η all_last , system efficiency is reduced.

[0081] If the bus voltage control unit input port 0 power is calculated using formula (4) to indirectly judge the system efficiency, set P d_last is the power value calculated last time. d <P d_last , system efficiency increases; P d >P d_last , system efficiency is reduced.

[0082] To verify the effectiveness of the present invention, Figure 1 The topology builds an experimental platform to conduct partial power conversion parallel battery storage system experiments. Figure 2 The positive direction of voltage and current is specified in [1]. The system's DC bus is rated at 125V, the rated power is 1kW, and the battery pack is rated at 85V. The system includes a DC bidirectional power supply, two series-connected battery modules, two SR-DAB converters, a DAB converter, and a host computer.

[0083] The system is initialized to the balanced bus voltage U od is 95V, the voltage adjustment step ΔU is 0.5V, the voltage adjustment time interval Δt is 15s, the counter upper limit s is 3, and the voltage adjustment direction Mode is 1.

[0084] Figure 6 The system operates in charging mode with a charging current of 4A. The balanced bus voltage is first set to 95V to ensure a DAB ratio of 1. At this point, the SR-DAB resonant cavity current is large, while the DAB inductor current is small. Once the automatic optimization algorithm is activated, the balanced bus voltage automatically adjusts, reducing the SR-DAB resonant cavity current and increasing the DAB inductor current. The system oscillates around the optimal efficiency value and automatically follows changes in the battery module port voltage, maintaining near-optimal efficiency.

[0085] Figure 7 This diagram shows how the automatic optimization algorithm re-searches for optimal efficiency after a change in system charging power. When the charging current changes from 3A to 5A, the balanced bus voltage automatically adjusts and oscillates around the new platform.

[0086] Figure 8The system operates in a discharge condition with a discharge current of -4A. First, the balanced bus voltage is set to 95V to ensure a DAB ratio of 1. At this point, the SR-DAB resonant cavity current is large, while the DAB inductor current is small. Once the automatic optimization algorithm is activated, the balanced bus voltage automatically adjusts, reducing the SR-DAB resonant cavity current and increasing the DAB inductor current. The system oscillates around the optimal efficiency value and automatically follows changes in the battery module port voltage, maintaining operation near the optimal efficiency value.

[0087] Figure 9 This diagram shows how the automatic optimization algorithm searches for the optimal efficiency value after the system discharge power changes. When the charging current changes from -3A to -5A, the balanced bus voltage automatically adjusts and oscillates at the new platform.

[0088] Figure 10 and Figure 11 The efficiency comparison chart shows that the system efficiency has been greatly improved regardless of charging or discharging conditions.

[0089] From the above examples, it can be seen that the automatic efficiency optimization algorithm can optimize the system efficiency by adjusting the balanced bus voltage value, and can automatically track the optimal efficiency value according to the changes in the SOC of the system battery module.

[0090] The above content is only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for optimizing the efficiency of a partial power conversion parallel battery energy storage system based on bus voltage regulation, characterized in that: The automatic optimization control algorithm for efficiency includes the following steps: Step 1: Initialize system settings, set the balanced bus initial voltage, voltage adjustment step ΔU, voltage adjustment time interval Δt, counter upper limit s, and balanced bus voltage adjustment direction Mode. Step 2: Select the voltage regulation direction according to the voltage regulation direction Mode. If Mode = 1, the balanced bus voltage increases by ΔU and waits for the voltage regulation time interval Δt; if Mode = 2, the balanced bus voltage decreases by ΔU and waits for the voltage regulation time interval Δt. Step 3: Evaluate the system efficiency. If the energy storage system operates in charging mode, the system efficiency is calculated using formula (1); if the energy storage system operates in discharging mode, the system efficiency is calculated using formula (2). If the system efficiency is evaluated indirectly, the system efficiency can be calculated by formula (4): the bus voltage control unit input port power P d Indirect assessment. Step 4: Determine the change in system efficiency. If the system efficiency increases, the counter is reset and the process goes to step 2. If the system efficiency decreases, the counter value is increased by 1 and the process goes to step 5. Step 5: Determine whether the counter is equal to the upper limit s. If so, switch the voltage regulation direction, reset the counter, and go to step 2.

2. A method for optimizing the efficiency of a partial power conversion parallel battery energy storage system based on bus voltage regulation, wherein step 2 is characterized by: Mode is the balanced bus voltage adjustment direction identifier. Mode = 1, the balanced bus voltage adjustment direction is rising; Mode = 2, the balanced bus voltage adjustment direction is falling. Based on the characteristic that the SOC of the battery module changes slowly, a longer voltage adjustment time interval Δt can be set to avoid frequent bus voltage adjustments and facilitate accurate evaluation of system efficiency.

3. A method for optimizing the efficiency of a partial power conversion parallel battery energy storage system based on bus voltage regulation, wherein step 3 is characterized by: (1) Directly evaluate system efficiency. The efficiency of the energy storage system in discharge mode is calculated as follows: The efficiency of the energy storage system in charging mode is calculated as follows: Among them, η j is the efficiency of the jth balancing unit, η o is the efficiency of the bus voltage control unit, η all is the system efficiency, P s and P isj are the total charge and discharge power of the battery and the input port power of each balancing unit. In formula (1), K j is the power of the jth battery module discharge power input to the DC bus through the balancing unit and the bus voltage control unit; in formula (2), K j It is the charging power of the DC bus output power flowing into the j-th battery pack through the balancing unit and the bus voltage control unit. G j is the transformation ratio of the jth subunit, calculated by formula (3): Among them, U bj is the battery module port voltage of the jth parallel branch, U isj is the voltage at the input port of the j-th parallel branch balancing unit. (2) Indirect evaluation of system efficiency can be determined by the bus voltage control unit input port power. The system efficiency of the energy storage system in charging or discharging mode is indirectly evaluated by the following formula: P d =U id *AND id (4) Among them, P d is the power of the bus voltage control unit input port, I id and U id are the bus voltage control unit input port voltage and current respectively. d <0, the system works in charging mode. If P d >0, the system works in discharge mode. When the charging and discharging power of the energy storage system remains unchanged, P d Can reflect changes in system efficiency.

4. A method for optimizing the efficiency of a partial power conversion parallel battery energy storage system based on bus voltage regulation, wherein step 4 is characterized by: If the system efficiency is directly calculated and evaluated using formula (1) and formula (2), set η all_last is the result of the last system efficiency calculation. If η all >η all_last , system efficiency increases; η all <η all_last , system efficiency is reduced. If the bus voltage control unit input port power is calculated using formula (4), the system efficiency can be indirectly determined by setting P d_last is the power value calculated last time. d <P d_last , system efficiency increases; P d >P d_last , system efficiency is reduced.

5. A method for optimizing the efficiency of a partial power conversion parallel battery energy storage system based on bus voltage regulation, wherein step 5 is characterized by: Balanced bus voltage regulation uses a counter to avoid system efficiency errors caused by voltage and current fluctuations at system ports, thereby ensuring the correctness of balanced bus voltage regulation. Specifically, the counter upper limit is set to s. If the efficiency drops s times consecutively, the balanced bus voltage regulation direction is changed, the Mode value is reset, and the counter is cleared.

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